There . There are a number of possible causes for a delayed period. Pregnancy is the most common one, and some women who don't realize they're pregnant have a miscarriage during their first trimester. This can cause an unusually heavy period.
Other potential causes for skipped or delayed periods include ovarian cysts, illness or severe emotional stress, certain medications (antidepressants, anti-convulsants, etc.), thyroid disease, obesity, excessive exercise, and dramatic changes in body weight. No matter what the cause may be, when you're not having periods, your ovaries still produce estrogen, which stimulates the lining of your uterus to thicken. When you finally do ovulate, your ovaries produce progesterone, which "withdraws" the lining of your uterus (which can get quite thick if you miss several periods), leading to a heavy period.
If you haven't already done so, call your doctor to discuss your heavy vaginal bleeding. He/she may want to see you to make sure you're not having a miscarriage or suffering from a medical condition, such as hypothyroidism, that's contributing to this problem.
Good luck!